The measuring accuracy of a force-measuring device, in particular a balance based on electromagnetic force compensation or on strain gauge technology, is influenced by numerous factors. Mettler-Toledo GmbH publication “Bauen Sie Ihre Qualität auf solidem Grund!”, January 2001, pp. 14-15 describes such technology and the influencing factors are described in “Wägefibel”, another Mettler-Toledo GmbH publication dated April 2001. Extraneous mechanical influences such as vibrations or shocks are particularly detrimental, which is the reason why filters for the removal of interfering signal components were provided already in balances with analog signal processing.
A balance, as disclosed in U.S. Pat. No. 4,860,839 to Reichmuth, uses an active low-pass filter that serves to suppress interference signals in the form of interfering alternating currents that are superimposed on the DC signal which is produced by the weighing cell and transmitted through a signal line to an analog/digital converter. The interference signals are separated out of the signal line at the signal output terminal of the weighing cell and, after a 180° phase shift by an inverter, they are returned to the signal line at the signal input terminal of the analog/digital converter, while the signal line itself between the branching-out and rejoining nodes contains only an ohmic resistance (i.e. a non-phase-shifting resistance). Consequently, the interference signals are canceled by signal components of corresponding amplitude but opposite phase.
U.S. Pat. No. 6,657,138, to Klauer, describes an electronic weighing transducer with a digital signal-processing unit wherein the DC signal component is determined from the output signal of the weighing transducer by means of a filter with a low-pass characteristic and wherein the weighing result is derived from the DC signal component. At the same time, a shock/vibration-dependent signal is determined, and the DC component of the measuring signal is modified dependent on the shock/vibration-dependent signal. This solution avoids drawbacks that occur in the solution disclosed in U.S. Pat. No. 5,665,941, to Wehhofer.
In Wehhofer '941, the time constant of the low-pass filter in a differential dosage-weighing balance is adapted depending on the interference signal. When large interfering disturbances are present, the time constant of the low-pass filter is made longer in order to achieve a stronger filtering effect. However, Klauer '138 indicates that this slows down the response of the weighing transducer to step changes of the weight while it improves the reproducibility of the measurements only to an insignificant extent. In addition, if the time constant is selected too large, this has the result of a long settling time with changes of the weighing load.
Further methods as well as balances suited for the use of the respective methods are described in United States published application 2004/0088342 to Aikawa and U.S. Pat. No. 6,271,484 B1 to Tokutsu, where signals produced by the measurement transducer are processed by means of variable digital filters.
The method described reference Aikawa '342 allows the characteristics of the filter that is used in a measuring system to be individually adapted to the oscillatory properties of the measuring system that is being controlled. The damping of the filter can therefore be increased as much as desired in a selected frequency range.
According to the method described in reference Tokutsu '484, a test is made whether the amplitude of the signal disturbances caused by vibrations lies within a permissible range. If this is not the case, the filter characteristic is changed until the signal disturbances fall into the permissible range again.
The last-described method in particular requires a high-volume computing effort and, due to the time constant of the regulation feedback loop, hardly permits a sufficiently fast adaptation to rapid changes of the amplitude of vibrations and oscillatory disturbances when they occur.
Of particular importance are the effects that load changes cause in the weighing system. A step change of the weighing load is followed by high-amplitude oscillations with a relatively fast decay. The objective for the filter device incorporated in the balance is to obtain the best possible damping of the oscillations that follow a step change of the weighing load and to simultaneously achieve a short settling time of the weighing system. A filter arrangement suitable for this purpose is shown in FIG. 1. With this device, the measuring signal is monitored in regard to a step change of the weighing load. After a step change has been detected, at least one filter parameter of the filter is reset and changed in accordance with a specific time profile, preferably in the form of an exponential function, so that the filter is opened after the load change has been detected and then closed again up to a given filter characteristic which is determined by the end value of the at least one filter parameter. Filter arrangements of this kind are realized preferably with digital filters.
Basic structures of digital filters are described in the book by U. Tietze and Ch. Schenk, entitled “Halbleiterschaltungstechnik” (“Semiconductor Circuit Design”), 11th Edition, 2nd Printing, Springer Verlag, Berlin 1999, chapter 21.3. FIG. 21.15 (page 1145) shows a digital filter which has a delay chain with n delay elements. According to this text at page 1144, the input signal of the delay chain is the result of the input signal delivered for example by a measurement transducer and the sum of all weighted intermediate values that are derived from the outputs of the delay elements. Accordingly, the output signal is the weighted sum of all intermediate values.
This has the consequence that the input signal with all changes and superimposed additions remains preserved in the filter stage for a relatively long time. It would therefore be desirable if the processing within the analog or digital filter stage included in essence only signal components that have a direct connection to the object being measured and if other signal components were completely suppressed.
In the pursuit of this objective, the suppression of disturbances in a force-measuring device is made more difficult by the fact that the useful signals that are caused for example by a change of the load and the disturbance signals that are caused for example by a mechanical shock or impact typically exhibit similar signal profiles, so that it is difficult to separate them from each other.
The embodiments disclosed herein have the objective to provide an improved method for the processing of the output signal of a measurement transducer, and to further provide a force-measuring device that operates in accordance with the disclosed method.